The objective of the proposed research program is a study of the mechanism of the transmural diapedesis of leukemic cells from their extravascular site of formation into the intravascular pool and their intra-and interorgan disposition. Shay's (cloro myelogenous leukemia and the myelogenous leukemia induced by ionizing irradiation will be used. The methodology includes: observations by transmission electron microscopy, by secondary emission and back-scattered electron scanning electron microscopy. The intra- and interorgan disposition and transmural migration will be studied by tracing and locating intravenously administered Shay leukemia cells labelled with H3 -thymidine by means of the scintillation counter and by radio-autography.